1. Field of the Invention
The present invention relates to a length measuring device.
2. Discussion of Related Art
Length measuring devices for determining the relative position between two objects which can be moved relative to each other are, for example, used for recording the position of linear axes without additional mechanical transfer elements in machine tools or in co-ordinate measuring machines. They include a measurement device and a scanning arrangement, which scans the measurement device as well as a carrier body, which supports the measurement device. Depending upon the conditions of application there is a distinction between open or sealed length measuring devices. Open length measuring devices are used where there is little or no contamination whereas sealed length measuring devices are preferably used on machines and installations with rough conditions of application with regard to contamination wherein an aluminum casing protects the measurement device and the scanning arrangement as well as the guide of the scanning arrangement from filings, dust and spray.
In order to maintain high measurement precision it is necessary for the length measuring device to follow the changes in length of the objects moved relative to each other caused by temperature changes. Furthermore, self-oscillations of the longitudinal measuring device, which can arise in certain circumstances due to oscillations on the objects moved, must be suppressed. If, for example, the length measuring device is used on a machine tool the oscillations occurring on the machine tool are transferred from the machine tool to the length measuring device. If the carrier body of the open or sealed length measuring device is connected at the ends only with one of the objects, as is in particular the case with shorter length measurements, where the middle area is not supported and tends to oscillate with its own resonance frequency when the object""s mounting area oscillates. These oscillations are transferred to the measurement device and can amongst other things lead to a situation wherein the scanning arrangement loses contact with the measurement device, which can lead to incorrect measurements.
In the same way, the absence of contact of larger areas of the contact area of the carrier body to be connected to the mounting area of an object leads to poor thermal connection so that temperature-dependent changes in length of the object do not lead to adequate changes in length of the carrier body.
A further problem is that carrier bodies manufactured from an aluminum profile have no preferred orientation for deformation or twisting in such a way that with front fixing of a carrier body on the mounting area of an object the contact area of the carrier body is either pressed against the mounting area, lies plane parallel against it or is directed away from the mounting area. In the latter two cases there is a danger of the carrier body not following the mechanical and thermal changes of the object and showing its own dynamic characteristics with regard to oscillations and thermally dependent changes in length.
From DE 25 05 587 C3, a length measuring device is known wherein a carrier body is fixed at the ends with fixing elements formed in a jointed way on one of the objects which can be moved relative to each other wherein the carrier body lies free between the fixing elements in such a way that oscillations are facilitated which influence the measurement precision in a disadvantageous way.
In order to suppress self-oscillations and to improve the temperature pattern of the carrier body it is known that additional securing devices can be distributed in the middle or over the length of the carrier body and the contact of the support area of the carrier body with the mounting area of the object is thereby strengthened. Screws, fixing clamps or adhesive substances are used as securing devices.
From DE 35 09 390 A1, a length measuring device is known wherein as an addition to the elastic fixing elements at the ends of the measuring device casing further fixing elements are arranged which are however fixed at a distance from the object to be measured in such a way that oscillations can likewise occur which lead to measurement errors. Furthermore, this step for strengthening the contact between the support area of the carrier body and the mounting area of one of the objects requires additional securing devices and therefore higher manufacturing costs as well as greater manufacturing resources in the connection of the length measuring device with the objects which are moved relative to each other. In addition, more space is needed to apply the additional securing device but such space is not available in many applications.
From DE 44 06 798 C2, a position measuring device is known wherein the measurement device is fixed on a carrier body which is fixed rigidly at one point and at least at one further point via elastic elements for translational bearing of the carrier body to one of the objects and is supported at least with a surface area on one of the objects. A highly elastic layer made from an adhesive layer with an anti-friction layer applied thereto is provided in this support area running in the measurement direction between the area of the carrier body and a mounting area of the object running parallel thereto. This step for improving the contact between the contact area of the carrier body and the mounting area of one of the two objects, which can be moved relative to each other does however require increased manufacturing and assembly resources.
Another example of a known length measuring device is shown in FIG. 1. The length measuring device records the position of two objects which can be moved relative to each other, for example for the measurement of the carriage displacement on a machine tool. The length measuring device has a measurement device 20 in the form of a gauge, which is fixed in a carrier body 22, which includes, for example, an extruded aluminum profile or aluminum profile produced in a die-casting process. A photo-electric or magnetic sensor arranged in a scanning carriage 24 of a scanning arrangement 26 scans the measurement device 20. The carrier body 22 and therefore the measurement device 20 are connected with one of the two objects which can be moved relative to each other whereas the scanning arrangement 26 is connected with the other one of the two objects in such a way that a relative change in position of the two objects is recorded.
In the example shown in FIG. 1, the carrier body 22 is closed at its front ends 28, 30 by end pieces 32 which are inserted in a shape-locking way in the profile of the carrier body 22. In the end pieces 32, oblong holes 34 or bores 36 perpendicular to the measuring direction, i.e. perpendicular to the length of the measurement device 20, are arranged. The oblong hole 34 is parallel to the plane of the measurement device 20 or perpendicular hereto as a bore 36. Fixing screws are put through the oblong hole 34 or the bore 36 and screwed with corresponding threads in the mounting area of one of the two objects in such a way that a secure connection is produced between the carrier body 22 and the mounting area.
FIG. 2 shows a schematic side view of an end-side connection of the carrier body 22 with the mounting area 38 through a force-locking or shape-locking connection of the front ends of the carrier body 22 with the mounting area 38 by end pieces 32.
With this form of connection,poor contact of the contact area 40 of the carrier body 22 with the mounting area 38 can arise in particular with drawn carrier bodies or carrier bodies produced in the extrusion press process for sealed length measuring devices in such a way that particularly in the middle area of the carrier body 22 the contact area 40 and the mounting area 38 are moved away from each other by the value A. This poor contact leads to the problems explained at the outset with the consequent incorrect measurements and determination of positions which are avoided by the solution according to the present invention.
It is an object of the present invention to provide a length measuring device of the type mentioned at the outset which when arranging the carrier body on the mounting area of an object ensures optimum contact of the carrier body with the mounting area without additional securing devices and without additional resources and space requirements.
This object is achieved according to the present invention through a length measuring device for determining a relative position of two objects, which can be moved relative to each other. The measurement device, includes a carrier body which supports the measurement device and is connected with a contact area with a mounting area of one of the two objects and with a scanning arrangement connected with another one of the two objects and scanning the measurement device along a measurement direction. Wherein a torque on an end face of the carrier body in the measurement direction acts on the carrier body in such a way that the contact area of the carrier body is pressed against the mounting area.
The solution according to the present invention creates optimum contact of the carrier body with the mounting area of one of the two objects, which can be moved relative to each other without additional securing devices and additional resources and space being necessary for arranging the carrier body on the mounting area of the object. The temperature and oscillation pattern of the length measuring device is therefore improved without increasing the manufacturing and assembly resources and therefore the costs of manufacturing and assembly.
The basis of the present invention is the recognition that a large-area contact of a carrier body connected at its front ends with the mounting area of an object can also be achieved in that at the front ends of the carrier body in the measuring direction a torque engages which produces a tight contact between a large part of the contact area of the carrier body and the mounting area of an object in the connection of the length measuring device with the objects which can be moved relative to each other.
The torque for large-area and tight contacting of the contact area with the mounting area of one or both of the two objects, which can be moved relative to each other, can be realized in difference ways.
A preferred embodiment includes a lever and a force producing the torque and working at a distance from the front end of the carrier body on the carrier body with which a large-area contact between the contact area of the carrier body and the mounting body of one of the two objects which can be moved relative to each other is produced.
Through the arrangement of a lever and the working of a force on the ends of the length measuring device the introduction of the torque for a large-area connection of the carrier body with the mounting area of the object is displaced into an area in which even with tight incorporation conditions perpendicularly to the measuring direction sufficient space is available which can work over the entire breadth of the carrier body which is preferably profiled.
The lever preferably includes a shoulder oriented to the mounting area of one of the objects and the force working on the carrier body is a tension element arranged on the side of the shoulder turned away from the front end.
Through a connection of the tension element with the mounting area of one of the objects the connection of the carrier body to the mounting area can be combined with the production of a torque, which reduces the manufacturing and assembly resources and therefore additionally the costs and ensures large-area positioning of the contact area in the middle area and a connection in the end areas of the contact area of the carrier body.
The shoulders are preferably integrated into end pieces which cover the front side of the carrier body and with which the casing of the length measuring device is closed at the end and with which the length measuring device is connected to the mounting area in particular by means of a screw connection.
The formation of the shoulder for producing the lever effect in connection with the tension element can be achieved in different ways.
In a first embodiment, the end piece covering the front side of the carrier body has a ledge projecting over the surface area of the carrier body lying against the mounting area and a screw connection between the carrier body or the end piece and the mounting area of one of the objects arranged at a small distance to the end piece.
The connection of the end piece with the carrier body can take place in a force-locking or in particular shape-locking way in such a way that the end piece has an insertion for inserting the end piece in a front opening of the carrier body/can be inserted perpendicularly to the measuring direction in the carrier body.
Alternatively, the shoulder can be realized by an end piece covering the front side of the carrier body and having a slightly angular mounting surface, which connects flush to the front cross-section of the carrier body and widens to the end of the end piece turned away from the front side of the carrier body. In association with a screw connection arranged at a small distance from the end piece between the carrier body or the end piece and the mounting area of one of the two objects which can be moved relative to each other, a simple lever arrangement is created which ensures large-area contact between the contact area of the carrier body and the mounting area.
A further possibility for realizing a shoulder of the front end of the carrier body includes arranging a web or several shoulders arranged beside one another perpendicularly to the measuring direction for example in the form of pinheads in the end area of the contact area of the carrier body which is to lie against the mounting area and providing at a small distance from the web or the shoulders screw connections as securing devices which connect the carrier body to the mounting area.